WPF CommandParameter is NULL first time CanExecute is called

I have found that the order in which I set Command and CommandParameter makes a difference. Setting the Command property causes CanExecute to be called immediately, so you want CommandParameter to already be set at that point.

I have found that switching the order of the properties in the XAML can actually have an effect, though I'm not confident that it will solve your problem. It's worth a try, though.

You seem to be suggesting that the button never becomes enabled, which is surprising, since I would expect the CommandParameter to be set shortly after the Command property in your example. Does calling CommandManager.InvalidateRequerySuggested() cause the button to become enabled?

I stumbled upon a similar problem and solved it using my trusty TriggerConverter.

public class TriggerConverter : IMultiValueConverter
{
    #region IMultiValueConverter Members

    public object Convert(object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        // First value is target value.
        // All others are update triggers only.
        if (values.Length < 1) return Binding.DoNothing;
        return values[0];
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotImplementedException();
    }

    #endregion
}

This value converter takes any number of parameters and passes the first of them back as the converted value. When used in a MultiBinding in your case it looks like the following.

<ItemsControl 
    x:Name="commentsList"
    ItemsSource="{Binding Path=SharedDataItemPM.Comments}"
    Width="Auto" Height="Auto">
    <ItemsControl.ItemTemplate>
        <DataTemplate>
            <StackPanel Orientation="Horizontal">
                <Button                             
                    Content="Delete"
                    FontSize="10"
                    CommandParameter="{Binding}">
                    <Button.Command>
                        <MultiBinding Converter="{StaticResource TriggerConverter}">
                            <Binding Path="DataContext.DeleteCommentCommand"
                                     ElementName="commentsList" />
                            <Binding />
                        </MultiBinding> 
                    </Button.Command>
                </Button>
            </StackPanel>                                       
         </DataTemplate>
    </ItemsControl.ItemTemplate>
</ItemsControl>

You will have to add TriggerConverter as a resource somewhere for this to work. Now the Command property is set not before the value for the CommandParameter has become available. You could even bind to RelativeSource.Self and CommandParameter instead of . to achieve the same effect.

I recently came across the same problem (for me it was for the menu items in a context menu), nad while it may not be a suitable solution for every situation, I found a different (and a lot shorter!) way of solving this problem:

<MenuItem Header="Open file" Command="{Binding Tag.CommandOpenFile, IsAsync=True, RelativeSource={RelativeSource AncestorType={x:Type ContextMenu}}}" CommandParameter="{Binding Name}" />

Ignoring the Tag-based workaround for the special case of context menu, the key here is to bind the CommandParameter regularly, but bind the Command with the additional IsAsync=True. This will delay the binding of the actual command (and therefore its CanExecute call) a bit, so the parameter will already be available. This means, though, that for a brief moment, the enabled-state might be wrong, but for my case, that was perfectly acceptable.

You may be able to use my CommandParameterBehavior that I posted to the Prism forums yesterday. It adds the missing behaviour where a change to the CommandParameter cause the Command to be re-queried.

There's some complexity here caused by my attempts to avoid the memory leak caused if you call PropertyDescriptor.AddValueChanged without later calling PropertyDescriptor.RemoveValueChanged. I try and fix that by unregistering the handler when the ekement is unloaded.

You'll probably need to remove the IDelegateCommand stuff unless you're using Prism (and want to make the same changes as me to the Prism library). Also note that we don't generally use RoutedCommands here (we use Prism's DelegateCommand<T> for pretty much everything) so please don't hold me responsible if my call to CommandManager.InvalidateRequerySuggested sets off some sort of quantum wavefuntion collapse cascade that destroys the known universe or anything.

using System;
using System.ComponentModel;
using System.Windows;
using System.Windows.Input;

namespace Microsoft.Practices.Composite.Wpf.Commands
{
    /// <summary>
    /// This class provides an attached property that, when set to true, will cause changes to the element's CommandParameter to 
    /// trigger the CanExecute handler to be called on the Command.
    /// </summary>
    public static class CommandParameterBehavior
    {
        /// <summary>
        /// Identifies the IsCommandRequeriedOnChange attached property
        /// </summary>
        /// <remarks>
        /// When a control has the <see cref="IsCommandRequeriedOnChangeProperty" />
        /// attached property set to true, then any change to it's 
        /// <see cref="System.Windows.Controls.Primitives.ButtonBase.CommandParameter" /> property will cause the state of
        /// the command attached to it's <see cref="System.Windows.Controls.Primitives.ButtonBase.Command" /> property to 
        /// be reevaluated.
        /// </remarks>
        public static readonly DependencyProperty IsCommandRequeriedOnChangeProperty =
            DependencyProperty.RegisterAttached("IsCommandRequeriedOnChange",
                                                typeof(bool),
                                                typeof(CommandParameterBehavior),
                                                new UIPropertyMetadata(false, new PropertyChangedCallback(OnIsCommandRequeriedOnChangeChanged)));

        /// <summary>
        /// Gets the value for the <see cref="IsCommandRequeriedOnChangeProperty"/> attached property.
        /// </summary>
        /// <param name="target">The object to adapt.</param>
        /// <returns>Whether the update on change behavior is enabled.</returns>
        public static bool GetIsCommandRequeriedOnChange(DependencyObject target)
        {
            return (bool)target.GetValue(IsCommandRequeriedOnChangeProperty);
        }

        /// <summary>
        /// Sets the <see cref="IsCommandRequeriedOnChangeProperty"/> attached property.
        /// </summary>
        /// <param name="target">The object to adapt. This is typically a <see cref="System.Windows.Controls.Primitives.ButtonBase" />, 
        /// <see cref="System.Windows.Controls.MenuItem" /> or <see cref="System.Windows.Documents.Hyperlink" /></param>
        /// <param name="value">Whether the update behaviour should be enabled.</param>
        public static void SetIsCommandRequeriedOnChange(DependencyObject target, bool value)
        {
            target.SetValue(IsCommandRequeriedOnChangeProperty, value);
        }

        private static void OnIsCommandRequeriedOnChangeChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            if (!(d is ICommandSource))
                return;

            if (!(d is FrameworkElement || d is FrameworkContentElement))
                return;

            if ((bool)e.NewValue)
            {
                HookCommandParameterChanged(d);
            }
            else
            {
                UnhookCommandParameterChanged(d);
            }

            UpdateCommandState(d);
        }

        private static PropertyDescriptor GetCommandParameterPropertyDescriptor(object source)
        {
            return TypeDescriptor.GetProperties(source.GetType())["CommandParameter"];
        }

        private static void HookCommandParameterChanged(object source)
        {
            var propertyDescriptor = GetCommandParameterPropertyDescriptor(source);
            propertyDescriptor.AddValueChanged(source, OnCommandParameterChanged);

            // N.B. Using PropertyDescriptor.AddValueChanged will cause "source" to never be garbage collected,
            // so we need to hook the Unloaded event and call RemoveValueChanged there.
            HookUnloaded(source);
        }

        private static void UnhookCommandParameterChanged(object source)
        {
            var propertyDescriptor = GetCommandParameterPropertyDescriptor(source);
            propertyDescriptor.RemoveValueChanged(source, OnCommandParameterChanged);

            UnhookUnloaded(source);
        }

        private static void HookUnloaded(object source)
        {
            var fe = source as FrameworkElement;
            if (fe != null)
            {
                fe.Unloaded += OnUnloaded;
            }

            var fce = source as FrameworkContentElement;
            if (fce != null)
            {
                fce.Unloaded += OnUnloaded;
            }
        }

        private static void UnhookUnloaded(object source)
        {
            var fe = source as FrameworkElement;
            if (fe != null)
            {
                fe.Unloaded -= OnUnloaded;
            }

            var fce = source as FrameworkContentElement;
            if (fce != null)
            {
                fce.Unloaded -= OnUnloaded;
            }
        }

        static void OnUnloaded(object sender, RoutedEventArgs e)
        {
            UnhookCommandParameterChanged(sender);
        }

        static void OnCommandParameterChanged(object sender, EventArgs ea)
        {
            UpdateCommandState(sender);
        }

        private static void UpdateCommandState(object target)
        {
            var commandSource = target as ICommandSource;

            if (commandSource == null)
                return;

            var rc = commandSource.Command as RoutedCommand;
            if (rc != null)
            {
                CommandManager.InvalidateRequerySuggested();
            }

            var dc = commandSource.Command as IDelegateCommand;
            if (dc != null)
            {
                dc.RaiseCanExecuteChanged();
            }

        }
    }
}

Hey Jonas, not sure if this will work in a data template, but here is the binding syntax I use in a ListView Context menu to grab the current item as a command parameter:

CommandParameter="{Binding RelativeSource={RelativeSource AncestorType=ContextMenu}, Path=PlacementTarget.SelectedItem, Mode=TwoWay}"

In .NET 7.0 RC1 this has been fixed.

(At least in a sense...)

It should now automatically reevaluate CanExecute() whenever CommandParameter changes, including upon initialization.

Although that does not prevent the initial call to CanExecute() when CommandParameter is still null, many ICommand implementations should already be handling that and it does render obsolete the obscuure and problematic XAML attribute ordering workaround / hack.

As intimated by @Daniel-Svensson in a GitHub comment:

the actual issue here is that ICommand.CanExecute is not reevaluated when the value bound to CommandParameter changes. Doing so would obviously be the correct behavior, since the command parameter is passed to CanExecute, so everyone expects this behavior intuitively.

And that is what is being fixed.

According to @pchaurasia14, a Sr. Engineering Manager for WPF at Microsoft:

This has been fixed in RC1 release. You may try it out. ... I meant, .NET 7 RC1.

At the moment the GitHub tracking issue #316 in the dotnet/wpf project is still listed as open. Maybe that is because it's scope was broader. But the code change CommandParameter invalidates CanExecute #4217 has been included in .NET 6.0 RC1. It was merged on July 21, 2022 and is included in the list of commits (scroll way down) for the RC1 release.

(However, I guess I don't understand how a commit made in 2022 is included in a release from 2021).

Its a long shot. to debug this you can try:
- checking the PreviewCanExecute event.
- use snoop/wpf mole to peek inside and see what the commandparameter is.

HTH,

The commandManager.InvalidateRequerySuggested works for me as well. I believe the following link talks about similar problem, and M$ dev confirmed the limitation in the current version, and the commandManager.InvalidateRequerySuggested is the workaround. http://social.expression.microsoft.com/Forums/en-US/wpf/thread/c45d2272-e8ba-4219-bb41-1e5eaed08a1f/

What important is the timing of invoking the commandManager.InvalidateRequerySuggested. This should be invoked after the relevant value change is notified.